LNB stands for "Low Noise Block Converter". It is what captures the digital signal from the satellite. The dish itself is a shaped mirror for a high frequency signal which gathered and reflected it to a LNB. The LNB is called dual because it is actually 2 LNBs in 1 block.

The purpose of LNB is to reduce frequency of the incoming signal (down convert) so that the Receiver Box (standard incoming frequency range is 950-1450 Mhz) can operate on it. The base frequency fb, generated by the local oscillator (that is a principle of every heterodyne receiver or any gadget where necessary to move frequency range), is mixed with the incoming signal frequency fs and  the difference frequency (absolute value of (fs-fb)) is passed on to cable toward receiver.

Info provided by California Amplifiers (see also specs for dual LNB or LNB for sat ‘A’ for III sat ODU).

In high-power DBS (Direct Broadcast Satellite) systems such as DirecTV, each channel is broadcast from the satellite on a particular transponder with either a "left hand" or "right hand" circular polarization. Each LNB on satellite dish can be tuned to either the odd transponders or the even transponders, so each receiver can receive roughly half the channels at any given time. The most important thing to understand about DSS systems is that the receiver box actually tells the antenna (LNB) which transponder it wants. The satellite receiver has a table (matrix downloaded from the satellite) of which channels are on which transponder (and on which satellite), so it can "tell" to the dish which polarization/transponder to select and deliver down the coax. It does this by placing a variable voltage on the cable. (See table below.) . (Hint for troubleshooting – if customer says that he can not see some channels – ask him to show you one of them – then go to a signal meter and receiver shows the transponder that carries that channel – it will give you an idea – which transponder do not work (not on all boxes)). There are 32 transponders (numbered 1-32) on the 101 degree satellite (main satellite), 3 transponders for sat 110 – 8,10,12: and 7 transponders for sat 119 – 22,24,26,27,28,29,30,32 – currently in use for DirecTV.

In setup with two receivers, each receiver is connected directly to an LNB. Based on the channel you want to watch, the satellite receiver feeds the LNB giving 18 or 13 volts, telling it whether it should be tuned to the even or odd transponders. So, even if you had compatible splitters (meant with needed frequency range and power pass ports), and you used those to split the cable coming from an LNB, you would have one receiver that wanted to tune the LNB to an odd transponder while the other receiver wanted to tune the LNB to an even, as a result it would be switched to for a receiver with a higher voltage. And both boxes may have channels with even transponders.

The DSS Multiswitch takes both inputs from a Dual LNB and lock them – one for 13 volts and the other for 18 volts and make possible to have 4 or 8 outputs. At any given moment of time depends of which channel selected on the receiver, so that particular output will be connected to certain input 13 or 18 volts. You can then run one piece of coax to a standard DSS receiver or two pieces of coax to a DirecTV TIVO or Ultimate TV receiver.  Here is simplified diagram of how Multiswitch works:

receiver.  Here is simplified diagram of how Multiswitch works:

Pic.2.

The above described multswitch can be used with only one LNB or – one satellite. In case if more satellites involved we need to give an additional parametr (signal) which will tell to a multiswitch that it needs to choose different satellite (or use different inputs). ‘Receiver1’ on output 1 plays the channel on the odd transponder and the rest of them which are 2,3,4 are playing the channels from even transponders.

This is a reason why you have to pull 2 separate lanes for a TIVO. You can not use things like splitter or (even more crazier idea I have seen) diaplexer to hook up more receivers to the dish.

22 kHz switches are used by DIRECTV for work with dishes carrying more than 1 satellite. The presence of a continuous 22 kHz 600 mV peak-to-peak signal from the IRD instructs the switch to select the secondary input. The primary input is always select (and used for sat 101) in the absence of the 22 kHz signal. Following scheme allows for only two signal sources (it might be sat 101 and 119).

pic.3.

In case if you have a Spanish customer with hard to find location for a receiption for 101 and 119 satellites you can use a multiswitch (like Zinwell 4 by 4 or 4 by 8) and install separate dishes for 101 and 119. When you connect the receiver to the multiswitch, the switch determines which of the two satellites the receiver needs to look at, depending if it needs to view odd or even transponders. When you change the channel on the other satellite, the switch then swaps your connection to the other LNB when needed. (In short – if customer doesn’t have a line of sight for both satellites 101 and 119 we can use two dishes and 4 by 4 or 4 by 8 multiswitch .)

TIP:
There are two types of multiswitches:
1)powered up
2)non powered up (passive)

Second type easy to check, because once you set on the receiver (in the option to check signal strength) transponders ‘26’ or ‘27’ on sat 119, the input for "22 Khz" activates and you can check presence of 13 and 18V. Same for satellite 101 any odd and any even transponder.
Unfortunately with powered multiswitches it is impossible so we have to use meter with "22 KHz" to check if LNB is working.

In case if more outputs needed - two Multiswitches can be connected together (cascade one to another or parallel ) to add more connections. The reliability really depends. There are many issues that you have to be aware of though....length of cable, quality of multiswitches and whether they are powered or non-powered may affect whether or not this will work. If you have an "triple sat" elliptical dish, it is a little more complex, but can be accomplished. As mentioned above, the receiver will send a 22khz tone (this tone is used by the multiswitch) to tell the multiswitch that it wants the 119 degree satellite. If you have two 4xn switches connected or a 4xn multiswitch connected to the built in multiswitch on a dish, the "2nd" one will never tell the "1st" one that it needs to see the 119 so the receivers connected to the "2nd" one would ONLY see the 101 satellite. There are switches like Zinwell, designated as "cascadable" that WILL send that 22khz tone upstream, thereby allowing you to connect it in series with another multiswitch (such as a built in one on the dish).

The other way is to get "tone generators" which go on the two cables designated for the 119 satellite. They go inline between the two multiswitches (or between dish and multiswitch if you have the built in multiswitch on the dish). These tone generators are about $10-$15 each and they put the 22khz tone on the line so that the 1st multiswitch (or built in one) sees the tone and puts that line over to the 119 satelllite.

Case with two 4 by 8 multiswitches has shown on the picture (5x16-PhaseIII-parallel). – Picture is provided by ‘eagleaspen.com’.

The most common means of conducting signals from one piece of equipment to another is coaxial cable. Coaxial cable is often referred to as simply "coax". Coax is available from many manufacturers and comes in a variety of sizes, shapes, colors, specifications and capabilities. The most commonly recommended "coax" type is RG6, but this designation actually represents a family of cables with widely varying electrical characteristics. Though similar in many ways, each cable group has its own various physical and electrical characteristics, which must be taken into consideration.

The RG reference is the cable specification for use as a "radio guide ",while the numerical value helps differentiate the specifications of each individual cable. Although each cable has its own number, characteristics, and size, there is no difference in the way these different numbered cables work.

Coax Construction
Common "coax" cable is circular. Each has a center conductor surrounded by dielectric insulating material, which in turn is covered by a braid to shield against electromagnetic interference (EMI). The outer covering is the "jacket".
The coaxial cable's two conductors are separated by a nonconductive or dielectric material. The outer conductor (braid) acts as a shield and helps isolate the center conductor from spurious electromagnetic interference. The outer covering helps physically protect the conductors.

Center Conductor
The center conductor is the primary means of carrying a signal. The center conductor comes in varying diameters, usually ranging from 14 gauge to 22 gauge. The structure of the center conductor generally is solid copper or copper- clad steel, designated as bare copper weld. Variation in the size of the center conductor has an overall effect on the amount of DC resistance offered by cable. Cables which contain large diameter center conductors have lower resistances than cables with smaller diameters. This decreased resistance of large diameter cable enhances the ability of a cable to carry a signal over a longer distance with better clarity, but, it is also more expensive and harder to work with.
For applications where the cable may move up/down or side-to-side, select cable that has a center conductor consisting of many small strands of wire. As the cable moves, these strands flex and resist wear due to fatigue better than a cable with a solid center conductor.

Dielectric Insulating Material
Surrounding of the center conductor is an evenly made dielectric insulating material which is mostly available in some form of polyurethane. This dielectric insulator helps determine the operating characteristics of coax cable by maintaining uniform spacing between the center conductor and its outer elements over the entire length of the cable. Although foam dielectric material offers the best performance, it can absorb moisture, which will change its electrical behavior. Because of its rigid properties, solid polyethylene maintains its shape better than foam and withstands the pressures of accidental pinching or crimping, but, this characteristic also makes it slightly more difficult to handle during installation. In addition, its loss/length attenuation factor is not quite as good as foam, which should be considered in long cable runs. Also quality of dielectric directly influences its impendence. (see further for simplified electrical model of coax cable). Impendence, in simple way – can be described as a resistance to a high frequency currents (or the signal) between central conductor and shield if you will consider the cable as a load for the source (antenna for instance). Impendence stability directly refers to the losses in cable. In order to transmit maximum energy from the source to the receiver, impendence of source, feeder (cable) and load must be the same. If the impedances are not matches, part of the waves in the cable will be reflected back on the cable connections distorting the outbound waves. When these reflected waves hit the wave generator, they are again reflected and mingle with the outbound waves so that it is difficult for the receiver to tell which waves are original and which are re-reflections.

The same thing happens when pulses are sent down the cable (digital modulation) - when they encounter impedance other than the characteristic impedance of the cable, a portion of their energy is reflected back to the sending end. If the pulses encounter an open circuit or a short circuit, all of the energy is reflected (except for losses due to attenuation - another subject). For other terminations, smaller amounts of energy will be reflected.

This reflected energy distorts the pulse, and if the impedance of the pulse generator is not the same as the characteristic impedance of the cable, the energy will be re-reflected back down the cable, appearing as extra pulses. Any connection sharp curve can cause a little mismatch and different frequencies will react differently on that. And only god knows what frequencies (in our case transponders) can be lost.

The interesting thing to notice is that this minimum loss does not directly yield line impedance: the line impedance depends on the dielectric constant of the dielectric. For air insulated line, the corresponding impedance is about 76.71 ohms, but if the line uses foam dielectric with a velocity factor of 0.8, then the impedance of minimum atten would be about 61 ohms. However, that minimum is a pretty broad one, and don’t start loosing a lot till value of impendence gets more than perhaps 50% away from the optimal impedance.

Foam-dielectric line with the same impedance and outer diameter as solid-dielectric line will have lower loss. That's because, to get the same impedance, the foam line will have a larger inner conductor, and that larger conductor will have lower RF resistance, and therefore lower loss. Thus for coax cables impendence will be determined by inner/outer conductor ratios and the dielectric constants of the material between conductors for coax cables.

More mismatches between the source's output impedance, the cable's characteristic impedance, and load's input impedance (it can be cable crushing or kinking, or install connectors installed improperly or improper connections or any barrels in line) woulf result – more reflections – more resulting power loss. And sometimes reflected power can damage the power source if lits of power is sent to the cable (e.g., a radio transmitter). So you need to be careful of impedance mismatches connections.

For these who is interested – look for a "Theory of Long Lines". Any line can be called "Long Line" if it’s length longer than the radio wave transmitted through that line. It is quite interesting and complicated subject and not considered here.

Braid or Shield
Wrapped around the outside of the dielectric material is a woven copper braid (shield), which acts as a second conductor or ground connection between the camera and the monitor. It also acts as a shield against unwanted external signals commonly called electromagnetic interference or EMI, which may adversely affect a signal.

The amount of copper or wire strands in the braid determine how much EMI it keeps out. DirecTV allows to use grade coax cables containing loosely woven copper braid have shielding coverages of approximately 60 percent and higher. These cables are suitable for general purpose use in applications where electrical interference is known to be low.

Aluminum or foil cable without a braid may add distortions into signal to such a point that signal quality may be far below the level required for proper system operation, especially over long cable runs, and therefore not recommended for use.

Outer Jacket
The last component comprising a coax cable is the outer jacket. Although other materials are used, polyvinyl chloride, or PVC, is commonly used in its construction. Available in many colors such as black, white, tan, and gray, the jacket lends itself to both indoor and outdoor applications.

This model reminds many Low-pass filters connected in series.

Inductance L depends on diametr, material and lentgth of cable.

Capacitance C depends on dielectric constant of dielectric and distance between central conductor and braid

Inductive reactancein Ohms is:

XL=2×π×f×L, where f – frequency, L – inductance in henries.

And capacitive reactance in ohms is:

XC=1/(2×π×f×C), where f – frequency, C – capacitance in farads.

Frm that we see that inductance increases it’s resistance on higher frequencies, and capacitance increases its conductivity on higher frequencies. Long story short – losses in cable on higher frequencies are significantly higher. That is why not any cable can be used for a satellite system.

In addition with higher frequency the ‘skin effect’ takes over. That is due mainly to the limited penetration of current into the inner conductor. With increasing frequency, the current penetrates less deeply into the conductors, and thus is confined to a thinner region of metal. Therefore the resistance, hence attenuation, is higher. It also can be caused partly by energy loss in the dielectric material. Taking above factors into consideration understandable why RG6 must be used for a satellite system.

In the old houses we still can meet homeruns made by RG59 cables.

RG59 is a lower grade of coaxial cable than RG6, consisting of a smaller center conductor, a smaller insulating dielectric, and a single outer shield. However, it delivers acceptable performance for CATV. RG-59 has a 22 AWG center conductor.

RG6, on the other hand, has a larger center conductor (18 AWG), a dual or quad shield (2 braids and 2 foils), and a much larger insulating dielectric.

The benefits of using RG6 cable include: more bandwidth, lower susceptibility to interference, and lower attenuation per foot. All of these characteristics allow it to handle a higher bandwidth than RG59 cable. RG6 delivers exceptional performance for CATV, satellite, and all other video applications and is considered the cable of choice for digital TV.

RG59 has electrical ‘voltage drop per foot’ to a direct current 0.06 V.

RG6 has electrical ‘voltage drop per foot’ to a direct current 0.04 V. (see the characteristics of the cables)

Voltage drop depends on resistance of the cable. Encyclopedia of Physics states that resistanceof a long cylinder or wire is given by:

R=ρ× L/A,

where L is a length; A is a cross section area or diameter; ρ – resistivity of the material.( Ohm/meter)

That is determines maximum length of cable between LNB and receiver (power source in that case) 125 feet.

125 feet × 0.04V/foot = 5 volts. (difference between 13V and 18V)

Technically since everything has a tolerance so that LNB seems to switch to the Left hand polarization even with lower voltage.